The signatories of the Paris climate deal have until the end of next year to ensure their commitments meet the agreement's cap on global warming. But what mechanism will be checking their work and be the judge of countries' climate plans?
The question is so sensitive that, for now, the answer is: the countries themselves.
While nations have agreed to a global target aimed at avoiding the most catastrophic impacts of climate change, fossil fuel and other greenhouse gas emissions continue to rise when they should be falling.
And there is no formal organization tasked with making sure individual nations are on track.
"There are no 'police' to check; this is a weakness of the process," climatologist Corinne Le Quere told Agence France-Presse (AFP).
Nearly 200 countries signed up to the landmark Paris deal in 2015, committing to halt warming "well below" two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels, preferably limiting it to 1.5C.
And in November at the COP 26 climate summit in Glasgow, nations called for all governments to ensure their emissions plans for this decade align with the Paris temperature goals, strengthening them if necessary, by the end of 2022.
But each country will effectively mark its own climate homework.
The process means countries can move forward "at the pace suited to their political system," said Le Quere.
So far, it has not pulled down emissions nearly fast enough.
At a global level, the United Nations' climate change body estimates that countries' 2030 emissions reduction plans will lead to warming of a devastating 2.7C.
A separate annual analysis by the United Nations Environment Program (UNEP), which looks at the gap between climate commitments and actual reductions in greenhouse gas emissions, drills down into slightly more detail.
It includes a particular focus on richer nations from the G-20, responsible for around 80 percent of emissions.
"We don't name and shame but we do point to the G-20 members that are definitely not on track" like Australia and Mexico, said Anne Olhoff, one of the authors of the UNEP report.
But going further would be "so political, I don't see it happening in a U.N. context to be quite honest," she told AFP.
In fact, experts doubt that nations would agree to any formal external scrutiny.
"We heard very clearly in Glasgow, countries like the USA say that they – and they alone – will determine what is a 1.5C pathway for their country," said Bill Hare, of the research group Climate Analytics.
And the United States is by no means the only one.
"I don't think the onus in this case is on the EU because we are on track to stay well within Paris," European Commission Vice President Frans Timmermans told AFP during the COP conference in Glasgow.
"We can prove it with facts."
There are a few independent analyses like Climate Action Tracker (CAT) which calculate countries' estimated trajectory – towards 1.5C, 2C, 3C, or worse.
Hare said CAT, which his organisation partners with, and other assessments will spark "a lot of discussion and disagreement" in 2022 as the deadline looms.
"There needs to be a certain amount of naming and shaming of countries for progress to happen," said Hare.
"At least the visible risk of being named and shamed helps some countries focus on what they need to do to do the right thing."
Even the most ambitious countries will have to "sharpen their arguments" to convince people that their numbers add up, said Lola Vallejo, from the IDDRI think tank.
Civil society, the media and even other nations will all push governments to do more.
"Countries that are not in line with the Paris agreement will feel they are in the hot seat. Peer pressure works," said one Western diplomat.
Even the most ambitious emissions cutters will not be able to rest on their laurels.
Olhoff said richer nations will likely face questions over "the historical emissions burden and of fairness and equity".
The Paris agreement underlines the principle of "common but differentiated responsibilities" according to national situations.
So some believe that wealthy countries, largely responsible for global warming, have a duty towards the poorest, and should therefore do more to achieve a "fair" contribution.
There are multiple ways to measure this: historical emissions, emissions per capita, carbon footprint that takes into account emissions generated by imported goods, aid to poor countries.
But essentially the key message remains the same, said Olhoff.
If they hope to curb global warming, she said, "all countries need to go back and see if there is anything we can do more and quicker."
